Apophis, nicknamed the "Doomsday Asteroid," was once considered a potential threat, but now scientists realize the chance of the asteroid colliding with Earth is negligible. NBC's Brian Williams reports.

Radar observations made during this week's close encounter with the asteroid Apophis have ruled out the risk of a catastrophic cosmic collision in 2036, NASA says. Experts say it'll be much farther away at that time than it is right now.

The crucial readings came on Wednesday when the space rock, which is thought to measure at least 885 feet (270 meters wide), approached within 9 million miles (14.5 million kilometers) of Earth. NASA is monitoring Apophis with its 230-foot (70-meter) Goldstone radio dish in California. Optical readings also have come in from the Magdalena Ridge Observatory in New Mexico and the Pan-STARRS observatory in Hawaii.

The bottom line? "We have effectively ruled out the possibility of an Earth impact by Apophis in 2036," Don Yeomans, manager of NASA's Near-Earth Object Program Office at the Jet Propulsion Laboratory, said today in the all-clear news release. "The impact odds as they stand now are less than one in a million, which makes us comfortable saying we can effectively rule out an Earth impact in 2036. Our interest in asteroid Apophis will essentially be for its scientific interest for the foreseeable future."

Jon Giorgini, who developed JPL's online Horizons database to keep track of solar system objects, would go even further. He says that according to calculations based on the Goldstone data, Apophis will probably pass by Earth at a distance of 36 million miles (58 million kilometers, or 0.39 AU), and absolutely no closer than 14 million miles (22 million kilometers, or 0.15 AU). "That is a very extreme minimum," he told NBC News. "Nothing else plausible can get you closer."

Apophis, a.k.a. 2004 MN4, created a huge splash when it was discovered in 2004 because the initial assessment of its orbit gave a 1-in-40 chance of Earth impact in 2029. That would be catastrophic: The space rock is big enough to wipe out a city if it struck land, or create killer tsunami waves if it splashed into the ocean.

Additional orbital data quickly eliminated the risk for 2029, but showed that it would pass within 20,000 miles (32,000 kilometers) of our planet at that time. That's so close that Earth's gravitational field will perturb Apophis' orbit. The experts worried that if the asteroid passed through a particular half-mile-wide zone in space, known as a "keyhole," its orbit would be perturbed just enough to set up a smash-up during the 2036 encounter. Fortunately, the latest observations indicate that Apophis will miss the keyhole by a long shot.

Did I just hear a cosmic sigh of relief?

UH / IA

The asteroid Apophis, highlighted here by a white circle, was discovered in June 2004.

There are still a few uncertainties surrounding Apophis: Astronomers don't yet have enough data to determine how the asteroid is spinning or how solar radiation is affecting its orbital path — a phenomenon known as the Yarkovsky effect. Giorgini said that even under the worst-case scenario, the effect won't push Apophis into a collision in 2036. But there could conceivably be other risky encounters in the decades or centuries ahead.

"There's a non-linear amplification that can really move it around more," Giorgini said.

Also, there are questions about Apophis' exact size. Just this week, readings from the European Space Agency's Herschel space telescope suggested that the asteroid may be nearly 20 percent bigger than previously thought. But that larger size estimate is based on the assumption that Apophis is a spheroid, and astronomers already know that it's elongated.

"We're not seeing that larger size in the radar data," Giorgini said.

By the end of next month, continued radar observations from Goldstone as well as the Arecibo Observatory in Puerto Rico should give astronomers a much better fix on Apophis' spin and its size. When those factors are fully accounted for, the Jet Propulsion Observatory will update its official risk assessment for Apophis — and could take this bad boy off the hit list for good.

Update for 6:30 p.m. ET: Clark Chapman, senior scientist at the Southwest Research Institute, weighed in on the current state of the asteroid hunt in an email:

"One thing you should be aware of, and might mention, is that the next Planetary Defense Conference, an every-two-year international meeting, will be held April 15-19 in Flagstaff, Arizona. ... Some presentations are already listed in the program, which should be finalized a week from now, which is the due date for abstracts.

"An interesting tie-in with the new observations of Apophis is that a similar thing happened with 2011 AG5 a few weeks ago, when observations with the huge Gemini telescope in Hawaii showed that it would, in 2023, miss the roughly 350-km-wide 'keyhole' and, therefore, not strike the Earth in 2040. Prior to these critical observations, the chance of a 2040 impact was unusually high (though still low in everyday terms) at 1 in 500.

"A point to be realized is that while the chances of impact in these cases are very low by ordinary standards, they aren't zero, and the consequences of an impact could be very terrible, so it is important to plan and prepare for the possibility of impact until it is ruled out.

"It was important to get these observations of AG5 in the autumn of 2012, because if it had turned out that AG5 was actually on an impact trajectory, it would have given us an additional year to mount a deflection mission and succeed in deflecting it from the 2023 keyhole. Without making a major observational effort with a very large telescope this autumn, the next routine observational opportunity wasn't until this coming autumn."

Update for 8:30 p.m. ET: One of NASA's experts on the asteroid threat and two former NASA astronauts have weighed in on the report about Apophis. David Morrison of NASA's Ames Research Center sent these comments via email:

"One possible angle is the recent proposal from [NASA Administrator] Charlie Bolden, based on a Keck study, that we retrieve a 7-meter carbonaceous near-Earth asteroid and bring it into lunar orbit. There are many questions about this idea, but the one I have in mind is our assumed ability, without Sentinel, to find 7-meter C-type asteroids in Earthlike orbits. If you can't find them, you can’t protect against them, or do anything with them as potential resources."

Now here's an email from Ed Lu, a veteran of two space shuttle missions and an extended stay on the International Space Station. Lu now serves as chairman and CEO of the B612 Foundation, which is planning to launch the Sentinel space telescope to track half a million near-Earth asteroids:

"While it is great that Apophis is much better understood, and we know it won't hit us in 2036, the greatest danger from an asteroid strike is from the ones we haven't yet found. Of asteroids larger than the one that struck Tunguska in 1908, we know of less than 1 percent of them. And as David Morrison points out, we can't protect ourselves from the unknown asteroids (or make use of them either). The B612 Foundation Sentinel Space Telescope is going to work on finding and tracking these asteroids."

And here are some comments from Apollo 9 astronaut Rusty Schweickart, who has played a key role in raising awareness about the threats and opportunities presented by near-Earth objects. It was Schweickart who warned in the wake of Hurricane Katrina that asteroids like Apophis could spark a much more devastating "cosmic Katrina":

"I'm hoping that you don’t follow the bad (surprisingly wide) precedent of stating that [the risk from] Apophis has been eliminated. Please look on the JPL risk page and especially the more detailed info and note that 1) The 2036 impact possibility is, while significantly reduced, still possible, and 2) that the 2068 impact possibility is now elevated ... to a level that exceeds what the 2036 impact was prior to this apparition.

"There’s certainly good news re the 2036 impact decreasing in probability ... but frankly it was 1 in 234,000 prior to the new observations ... not exactly an impact probability to worry one. (There are many NEOs with higher impact probability ... but no one pays attention to them ... they aren't the 'poster child' that Apophis is.) My personal reaction was one of surprise that the new 2036 impact was not zero!

"But/And ... there are more radar observations to integrate in ... as well as optical tracking both now and for the next several years. Apophis isn't going away ... the impact possibilities are simply shifting around a bit with refinement of the tracking data. 2036 is now less probable; 2068 is now more probable (but still very low).

"Until JPL and the other guys get more data (enough to really define the Yarkovsky effect), we really won’t be able to get definitive data for longer time scales that we can rely on."

JPL's Giorgini said the risk assessment that Schweickart mentioned won't be full updated until after Goldstone and Arecibo finish their observational campaign in mid-February — so there may still be a non-zero risk listed until then. But Giorgini is confident that the 2036 risk will disappear when all the observations are factored in. (As of this writing, the estimated risk of collision is listed at 1 chance out of 10,989,000.) But you're right, Rusty: In order to eliminate the risk completely, astronomers will have to get more data about Apophis' physical characteristics. And then there are all those other unknown killer asteroids that might be out to get us...